The present invention relates generally to modems. More particularly, the present invention relates to multimode modems with automatic negotiation of operational mode.
BACKGROUND OF THE INVENTION
Secure telephony devices use integral modems to transfer an encrypted digital bitstream representing the user""s voice or other data across standard telephone networks. The most widely built and used secure telephone is the STU-III (Secure Terminal Unit, third generation) which was developed for the U.S. Government. A typical STU-III supports data transfers up to 9600 bps, and encodes and encrypts speech using federal-standard voice coders. The STU-III modem complies with a unique signaling plan, known as Future Secure Voice System (xe2x80x9cFSVSxe2x80x9d) Terminal Performance specification 210 (xe2x80x9cFSVS 210xe2x80x9d), which was developed for the National Security Agency.
Commercially available modems, which offer performance well in excess of 9600 bps, are based on standards developed by the International Telecommunication Union (ITU), which are documented in the ITU""s V.series recommendations. Commercial modems complying with Recommendations V.32, V.34, and V.90 are the most prevalent.
In certain applications, it is desirable that a single device be able to communicate interchangeably with either a secure modem or a commercial modem. It is well known, however, that the modem characteristics specified in FSVS-210 for STU-III interoperation are not compatible with the ITU V.series modem. Although a single device could conceivably communicate with both STU-III and ITU V.series modems with a priori knowledge of the far-end modem""s modes of operation, such a solution is impractical since the characteristics of a far end device usually are not known. It would be advantageous therefore, if a single device could communicate with both STU-III and ITU V.series modems without a priori knowledge of whether the far-end modem is a commercial modem or a secure modem, or of the far-end modem""s operational mode. Thus, there is a need in the art for a multimode modem that can automatically determine and negotiate operational mode of a far-end modem.
The present invention satisfies these needs in the art by providing apparatus and methods for determining an operational mode of a far-end modem.
Where the multimode modem is a call initiator, a method according to the present invention for identifying a far-end modem type includes transmitting a V.8 ANSam tone to the far-end modem, receiving a response signal from the far-end modem in response to the transmitted V.8 ANSam tone, and determining from the response signal whether the far-end modem is a commercial (e.g., V.series) modem or a secure (e.g., FSVS) modem.
If the response signal is a V.8 CM tone, then the far-end modem is determined to be a V.8 modem. If, on the other hand, the response signal is not a V.8 CM tone, but rather, has a nominal frequency of about 1800 Hz, then the far-end modem can be either a V.32 modem or a secure modem.
If the response signal has a nominal frequency of about 1800 Hz, but does not include phase shifts, then the far-end modem is determined to be a V.32 modem. If, on the other hand, the response signal includes phase shifts, then the far-end modem is determined to be a secure modem.
If the far-end modem is determined to be a secure modem, then an operational mode of the far-end modem can be determined from the response signal. If the response signal includes phase reversals, then the far-end modem is determined to be an FSVS modem in alternate mode. If the response signal includes a 128 dibit gap, then the far-end modem is determined to be an FSVS modem in half-duplex mode. If the response signal does not include either phase reversals or a 128 dibit gap, then the far-end modem is determined to be an FSVS modem in interoperable mode.
This invention is possible since a commercial V.series modem is looking for energy at 1800 Hz. Since the transmission of an 1800 Hz tone with 45-degree phase reversals would violate V.32 specifications (which require a pure 1800 Hz tone), the V.series initiator will ignore the 45 degree phase shifts present in a P1800 Hz tone, and consider the tone to be the expected xe2x80x9cAAxe2x80x9d tone. Also, the V.series initiator ignores the phase reversals in the xe2x80x9calternate modexe2x80x9d P1800 Hz tone, which occur in the first 80 msec of reply, since it looks for phase reversals only after it transmits xe2x80x9cACxe2x80x9d, which can occur no sooner than 155 msec after the start of the P1800 Hz tone.
Where the multimode modem is a call responder, a method according to the present invention for identifying a far-end modem type includes transmitting to the far-end modem a P1800 Hz tone with phase reversals, receiving a response signal from the far-end modem in response to the transmitted P1800 Hz tone, and determining from the response signal whether the far-end modem is a commercial modem or a secure modem.
An incoming channel is simultaneously monitored for the presence of a V.8 ANSam tone, and for energy at either 1800 Hz or 2100 Hz. If a V.8 ANSam tone is present in the incoming channel, then the far-end modem is determined to be a V.8 modem. If 1800 Hz energy is present in the incoming channel for at least about one second, then the far-end modem is determined to be an FSVS modem operating in half-duplex mode. If 2100 Hz energy is present in the incoming channel for at least about one second, then the far-end modem can be either a V.32 modem, or an FSVS modem operating in either interoperable mode or alternate mode.
If the response signal includes a V.32 AC, then the far-end modem is determined to be a V.32 modem. If, on the other hand, the response signal includes an FSVS Message A, then the far-end modem is determined to be an FSVS modem in alternate signaling mode. If the response signal includes neither a V.32 AC nor an FSVS Message A, then the far-end modem is determined to be an FSVS modem in interoperable mode.
Apparatus according to the invention include computer-readable media having stored thereon computer executable instructions for performing any of the inventive methods.